Lifting and moving large flat plates such as manhole covers is not only physically difficult but also hazards the risk of injury to those involved. A typical manhole cover is two to three feet in diameter, about one inch thick and, more importantly, weighs between 180 and 360 pounds. Open grid storm drain grates are of similar thickness and weight although usually rectangular in shape. This awkward size and weight has caused many a serious injury when only a pinch or pry bar produced by manual labor. An example of an improved pry bar intended to ease the initial lifting is shown in U.S. Pat. No. 3,837,622 by Gale.
A variety of configurations have received attention over the years to simplify plate handling. Some require special adaptation of the cover to match the tool such as the edge groove and tool hook arrangement of U.S. Pat. No. 4,482,182 by Mortensen and the threaded engagement devices of U.S. Pat. Nos. 3,198,362 by Berg, 4,157,810 by Haller et al, and 4,365,925 by Girtz. Such devices are useless for covers not modified to mate the engaging structure. In addition, dirt and debris from normal usage of the covers impedes use of the device even if the cover is suitably modified.
Others have suggested use of L-shaped arms for insertion into holes in the covers which are then forced or pivoted under the lower surface of the plate for lifting. Examples are shown in U.S. Pat. Nos. 4,126,297 by Barnes, 4,488,706 by Kono, 4,653,728 by Mochizuki et al and 4,662,607 by Mochizuki et al. The necessarily small size of the transverse lip of the L-shaped member along with hazards from wear of that lip from use seriously discount these devices as adequate solutions. In addition, they too suffer from problems of debris plugging the holes and, of course, are useless for covers that have no holes. U.S. Pat. No. 3,275,299 by Meshew, although another version of the L-shaped arm approach, additionally suggests configuring a lower ground engaging cam surface to aid in raising the cover after it is engaged.
A wide tongue for sliding under a dockboard and opposed parallel surfaces formed by inverted L-shaped cutaway stops is shown in U.S. Pat. No. 3,257,018 by Miles. While the Miles type device may represent a reasonably acceptable transporter for wide plates like dockboards, it is not an acceptable configuration for use with manhole covers and the like wherein the edge available for gripping is constrained in width. That is, use of a Miles type hand truck for moving manhole covers is extremely hazardous as the cover tends to slide laterally against the flat, parallel upper and lower surfaces of the tongue and cutaway stops.